Method and apparatus for controlling operation of forced flow critical and supercritical pressure steam generators



Feb. 2, 1965 P. PROFOS 3,168,075

METHOD AND APPARATUS FOR CONTROLLING OPERATION OF FORCED mow CRITICAL AND SUPERCRITICAL. PRESSURE STEAM GENERATORS Filed Aug. 21, 1961 4 Sheets-Sheet 1 Jn van {0 r: PA UL P/porcs Feb. 2, 1965 P. PROFOS 3,158,075

METHOD AND APPARATUS FDR CONTROLLING OPERATION OF FORCED FLOW CRITICAL AND SUPERCRITICAL PRESSURE STEAM GENERATORS Filed Aug. 21, 1961 4 Sheets-Sheet 2 Jn venfor: PHI/L. Beams 5 Feb. 2, 1965 Filed Aug. 21, 1961 P. PROFOS METHOD AND APPARATUS FOR CONTROLLING OPERATION OF FORCED FLOW CRITICAL AND SUPERCRITICAL. PRESSURE STEAM GENERATORS 4 Sheets-Sheet 3 3/7 vemor': P4 UL PPOFOS.

4 Sheets-Sheet 4 P. PROFOS PRESSURE STEAM GENERATORS FORCED FLOW CRITICAL AND SUPERCRITICAL Feb. 2, 1965 METHOD AND APPARATUS FOR CONTROLLING OPERATION OF Filed Aug. 21, 1961 mwm w & @Wh i P Zgvenfor':

AUL POP-O6.

United States Patent Dfifice 3,l68,b75 Patented Feb. 2, 19%5 3 168 i375 METHOD AND APPAKRA'E US FGR CQNTRULLlN-G GPERATHSN 6F FORCED FLQW CRRTHIAL AND dillPERCRlTlCAL PRESSURE STEAM GENER- ATURS Paul Profos, Winterthur, Switzerland, assignor to Sulzer lFreres, 3A., Winterthur, Switzerland, a corporation of Switzerland Filed Aug. 21, 1961, Ser. No. 132,376 Claims priority, application Switzerland, Aug. 21%, 196i arse/es ll Claim. (Ci. 122 451) The invention relates to a method and apparatus for controlling the operation of and more particularly the supply of feedwater t forced flow steam generators suitable for operation at least at the critical pressure of water.

The supply of feedwater to forced flow steam generators is usually controlled in response to the temperature of the operating medium somewhere along its path through the steam generator. A preferred location for measuring the temperature of the operating medium for the purpose of controlling the rate of feedwater supply is where a relatively small change of heat content of the medium causes a relatively great change of the temperature of the operating medium. If the steam generator is operated at, say, 300 atmospheres a small change of heat supply causes only a small change of the temperature of the operating medium at the outlet of the tube section of the steam generator wherein the water is evaporated and slightly superheated to about 415 C. At the outlet of a first superheater section where the temperature of the superheated steam of 300 atmospheres is around 465 C. the same change of heat content of the steam causes a much greater change of the steam temperature. A feedwater control in response to the temperature of the superheated steam at the outlet of a first superheater section is therefore more sensitive and more effective than a feed water control which is responsive to the temperature of the operating medium leaving the evaporating section of the steam generator, if the generator is operated at a relatively high pressure, for example 300 atmospheres.

if the steam generator is operated at a relatively low pressure, for example 100 atmospheres, a small change of the heat content of the operating medium causes a great change of the temperature of the medium already at the outlet of the evaporator where the steam temperature is around 315 C. and a satisfactory control of the feedwater supply can be effected in response to the temperature of the operating medium leaving the evaporating tube section, if the steam generator is operated at 1G0 atmospheres. It is, of course, desirable to control the rate of feedwater supply from a point of the tubular heating surface of the steam generator which is as close as possible to the inlet of the steam generator to reduce the time lag between the moment when the feedwater supply is changed and the moment when the temperature of the operating medium responds to the change of the feedwater supply.

It is an object of the invention to provide a method and means for controlling the feedwater supply to a forced flow steam generator, which is designed for operating at subcritical as well as supercritical pressures, in response to the temperature of the operating medium while it passes through the steam generator at a, point which is as close as possible to the inlet of the steam generator and at which point a relatively small change of heat content of the operating medium is accompanied by a relatively great change of the temperature of the operating medium, the method according to the invention including changing the point in the path of the operating medium through the steam generator Where the temperature of the operating medium is taken for controlling the rate of feedwater supply to the steam generator upon a change of the pressure at which the steam generator operates.

In a preferred embodiment the rate of feedwater supply to a forced flow steam generator is controlled in response to the temperature of the operating medium at the outlet of a tube section of the steam generator wherein the water is converted to steam as long as the generator opcrates at subcritical pressure and is controlled in response to the temperature of the steam in a tube section downstream of the evaporating tube section, if the steam generator operates at least at the critical pressure of Water.

The apparatus according to the invention includes means for controlling the rate of feedwater supply to a forced flow steam generator adapted to operate at subcritical, critical, and supercritical pressure of water, in response to the temperature of the operating medium at the outlet of the evaporating tube section of the generator when the latter operates at subcritical pressure and in response to the temperature of the steam in a tube section at a point which is downstream of the evaporating tube section when the generator operates at least at the critical pressure, changeover means responsive to the pressure of the operating medium being provided for changing the control of the rate of feedwater supply from a control in response to the temperature of the operating medium at the outlet of the evaporating tube section to a conrol in response to the steam temperature downstream of the evaporating tube section upon a change of the pressure of the operating medium from subcritical to supercritical pressure.

The method and apparatus according to the invention facilitate starting and cooling a forced flow critical pressure steam generator. They also permit temporary operation at subcritical pressure, for example for desalting the steam generator if, for example, due to leakage of a condenser, salt has come into the operating medium of the steam generator. During the period when the generator is operated at subcritical pressure the feedwater supply is controlled in response to the temperature at the outlet of the evaporator. If the salt concentration in the operating medium of the steam generator has been reduced to a permissible value, the pressure in the steam generator is increased to at least the critical pressure whereupon the rate of feedwater supply control is automatically switched from one responsive to the outlet temperature of the evaporator to the outlet temperature of a superheater downstream of the evaporator.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claim. The invention itself, however, and additional objects and advantages thereof will best be understood from the following description of embodiments thereof when read in connection with the accompanying drawing, wherein:

PEG. 1 is a schematic illustration of a steam power plant including a forced flow steam generator and a change-over apparatus for changing the feedwater regulation from starting-up operation of the plant to normal operation of the plant and conversely.

FIG. 2 is a schematic illustration of a steam power plant including a forced flow steam generator and a modification of the automatic feedwater regulation shown in FIG. 1.

FIG. 3 is a part-sectional diagrammatic illustration of a control mechanism as used in the plant shown in FIG. 2.

PEG. 4 is a diagrammatic illustration of an electric change-over apparatus.

FIG. 5 a schematic illustration of a steam power plant including a forced flow steam generator and provided with another modification of a feedwater regulation.

Referring more particularly to FIG. 1 of the drawing, numeral 1 designates a feedwater reservoir wherefrom feedwater is pumped by a feed pump 10 to a steam-heated feed water heater 11 into a feed pipe 13 provided with a feedwater regulating valve 12 and therefrom into an economizer 2013 a forced flow steam generator. 7 The latter includes a tube section 3 wherein water is evaporated, if the pressure in the steam generator is below the critical pressure of water which is the case, for example, when starting and when cooling off the steam generator. If the steam generator is operated at least at the critical pressure water is heated in the tube sections 2 and 3 and is converted into steam without ebullition in the tube section 3. A Water separator 14 is interposed in a pipe connecting the tube section 3 to a tube section 4. The latter forms a first superheater delivering superheated steam into asecnd superheater wherefrom the steam passes through a third superheater 6. The latter discharges into a steam main 15 connected to a turbine 8 which exhausts into a condenser 9. The condenser 9 is connected by means of a pipe 16 containing a condensate pump 17 and two stea. heated preheaters 18 and 19 to the feedwater reservoir 1.

A device 2t) measuring the rate of flow m of feedwater is connected to the feed pipe 13 downstream of the feedwater regulating valve 12. The device 20 produces a signal fed into a control apparatus 22 having an integral (1) characteristic. The output of the regulator 22 is conducted by means of a conduit 23 to a motor operator 12.

A pressure (p) responsive device 24 is connected to the pipe connecting the tubular heating sections 2 and 3 and produces a signal which is fed through a conduit 25 into a device 26 which is connected by means of a signal conduit 27 to a change-over device 28 which will be described later.

A conventional multithermostat 29 is connected to the outlet ends of the tubes forming the heating section 3, the thermostat measuring the temperatures of the operating medium in each or" the tubes of the section 3 which are arranged in parallel relation with respect to the flow of the operating medium. The device 29 produces signals corresponding to the highest temperature (t sensed by the multithermostat at any time. A suitable device is disclosed in Patent No. 2,800,887. The signal produced by the device 29 is conducted through a conduit 31 to an adding device 31. The latter is connected through a signal conduit 33 to the change-over device 28 and through a conduit 45 to a device 32 for controlling the output of the steam genera-tor. The device 31 adds up the signals arriving through the conduits 3d and 45 and feeds the total into the conduit 33 as indicated by plus signs in the drawing.

A rate of flow (m) measuring device 34 is connected to the steam conduit between the separator 14 and the first superheater section 4. The device 34 produces signals which are conducted through a conduit 35 to a proportionally (P) acting regulator 36. The output signal of the latter is conducted through a conduit 37 to an addition device 33 wherein this output signal is added to the output signal of a regulator 46, described later, the total being fed through a conduit 39 into the l-regulator 22 as indicated by signs in the drawing.

A temperature (2) measuring device 40 is interposed between the superheater sections 4 and 5 and produces signals which are conducted through a conduit 41 to an addition device 42 wherein signals arriving through a signal conduit 43 from the output control device 32 are added, the total being conducted through a signal conduit 44 to the change-over device 28 as indicated by plus signs in the drawing. The output control device 32 feeds signals corresponding to the desired steam temperatures through the conduits 43 and 45 to the addition device 42 and 31 respectively, wherein the signals corresponding to the desired temperatures are superimposed on the signals produced by the temperature sensitive devices 29 and 40. The resulting signals are conducted through conduits 44 and 33, respectively, to the change-over device 28 which, depending on its position, conducts one of the signals to a 'ously therewith and in the same sense.

proportional and integral (PI) regulator 46 whose output is conducted through a signal conduit 47 to the addition device 38.

During normal operation of the steam generator, i.e., when the generator is operated at least at the critical pressure of water and at corresponding high steam temperatures, the pressure sensitive device 24 actuates the changeover device 28 via the device 26 to conduct the signal produced by the temperature measuring device 443 to the Pl-regulator 46 whose output signal is conducted through the conduits 47 and 39 to the I-regulator 22 determining the set point thereof for controlling the rate of flow of feedwater to the steam generator. The I-regulator 22 is also influenced by a preimpulse arriving from the addition device 33 through the signal conduit 39. This preimpuise is produced by the signal produced by the rate of flow measuring device 34 and conducted through the P-regulator 36 and the signal conduit 37 to the addition device 38.

VJhen starting or cooling 01f the steam generator, or when desalting the steam generator the pressure measured by the device 24 is below the critical pressure and produces a signal which actuates the change-over device 28 so as to connect the signal conduit 33 to the PI-regulator 46 and to stop flow of the signals arriving from the conduit 44. Under these conditions the rate of flow of feedwater supplied to the steam generator is controlled in response to the temperature of the operating medium at the outlet of the evaporating heating surface 3. In this case the signal produced by the multithermostat 29 which is conducted through the PI-regulator 46 acts as set point signal in the I-regulator 22 which controls the feedwater regulating valve 12. The signals produced by the rate of flow measuring device 34 cooperate in the same manner as has been described before when the change-over device 28 was connected to the signal conduit 44.

The arrangement of the steam power plant shown in FIG. 2 is fundamentally the same as that shown in FIG. 1. in lieu of the multithermostat 25) shown in FIG. 1 a single thermostat 5% is connected to a tube 63 of the tubular heating section 3'. The tube 63 which acts as a measuring tube is provided with a throttling device 51 which is so adjusted that the rate of fiow through the tube 63 is somewhat less than the rate of flow through the other tubes of the heating section 3' which are connected in parallel relation with the tube 63 with respect to the flow of operating medium through the tubes. As a result slightly superheated steam of the temperature (t) emerges from the tube 63.

When in the embodiment of the invention shown in FIG. 2 the signal produced by the thermostat controls the feedwater regulation the set point of the PI-regulator 46' is influenced by a signal corresponding to the moisture content of the steam. For this purpose the separator 14 is provided with a conventional water level control whereby a signal is produced in a transmitter 52 which signal corresponds to the elevation (11) of the water level in the separator and acts on a proportional (P) regulator 53 which controls a blowdown valve 55 in a blowdown pipe 54. The P-regulator 53 is connected by means of a signal conduit 55 to an integration (I) device 57 whose output is conducted through a signal conduit 53 to a change-over device 59 which is provided in addition to the change-over device 28 and is actuated simultane- The integration device 57 is also connected through a signal conduit 60 containing a correcting device 34' to the rate of flow (in) measuring device 34. The correcting device 34 transforms the signal corresponding to the rate of flow of steam from the separator 14 to the first superheater i so that the transformed signal can be compared in the integration device 57 with the signal produced by the P-reguiator 53 which corresponds, to the actual rate of flow of the blowdown water. The signal corresponding to the result of the aforesaid comparison is conducted through the signal conduit 53 to the change-over device 59. The latter also is connected to a signal conduit 61 which receives signals from a set point signal transmitter, not shown. The output of the changeover device 59 is conducted through a signal conduit 62 as set point to the Pl-regulator 46.

The rate of flow control circuit including the parts 29, 21 and 22 shown in FIG. 1 may also be included in the embodiment of the invention shown in FIG. 2.

It is not absolutely necessary to provide the additional change-over device 59. By connecting the conduits 58 and 61 to the conduits 6% and 70, respectively, similar to the arrangement shown in PM}. 1, the additional changeover device can be avoided. Whether one or two change over devices are desired depends on the use of non-linear" regulating elements.

In the implementation of the control system by hydraulic means shown in FTG. 3 a rod 64 made of a material having a small temperature expansion coefficient, for example lnvar, is operatively connected to the measuring tube 63 of the steam generator section 3 which measuring tube is provided with a throttling device 51. The rod 6 5 forms part of a temperature measuring device fit) and has one end rigidly connected to the measuring tube 63, the second end of the rod being connected to a two-arr; lever 65 one end of which is swingably connected to the measuring tube 63, the other end being pivotally connected to a rod 6'7 which is yieldably connected by means of a spring 66 to a piston as. The latter is movable in a cylinder provided with ports for supplying and relieving a fluid under pressure and connected by a pipe 69 to the changeover device 2?.

Downstream of the first superheater section t a temperature measuring device 43 of the type of the aforeclescribed measuring device 59 is provided. Pressure fluid controlled by the device it) is conducted through a pipe iii to the change-over device 28.

The fluid in the pipe acts on a piston 71 and the fluid in the pipe it? acts on a piston 72. Piston rods 73 and 74 connected to the pistons '71 and 72, respectively, are connected to a guide element '75 for guiding a slide 76 which is slidable on the element 75. One end of a lever 77 is pivotally connected to the slide 76 and is also pivotally connected to one end of a push rod '78. The lever 77 can be rocked by means of a rod 79 connected to a piston 8'0 which moves in response to the pressure of the operating medium between the economizer 2 and the evaporating tube section 3', a pipe 81 being provided for transmitting the pressure. The lower end of the push rod 78 has a flat surface 82 placed opposite the outlet end of a bore 83 in a piston 84 which is connected by means of a spring to a piston as. The piston 84 has three cylindrical coaxial portions of different diameters. The center portion which has the greatest diameter is provided with a bore 87 connecting the spaces in a cylinder 88 on either side of the piston. The end walls of the cyl nder 88 have apertures attording axial movement of the small diameter parts of the piston 84. The space in the cylinder 88 adjacent to the lower end wall thereof is connected by a pipe 89 to a source of compressed air. The air flows through the bore 87 and through a diametrical bore in the upper small diameter portion of the piston 84 into the bore 83 wherefrom it escapes, depending on the relative positions of the flat surface 82 and the upper end of the piston 84. Due to the varying relative positions of the piston 8-5 and the surface 82 the throttling effect at the outlet of the bore 83 is varied and the piston 84 assumes a position in which the pressures acting on both sides of the piston are equal. The piston 84 follows the movements of the push rod 78 without applying an appreciable force thereto.

The piston 8-6 moves in a cylinder connected by a pipe 94) to a pilot valve M forming part of the Pl-regulator 46'. The valve 91 is connected by two valved pipes 92 to the spaces in a cylinder 93 at the opposite sides of a piston movable therein and forming the integral part of the regulator. The pilot valve 91 is in the form of a piston valve moving in a cylinder whose space at one end of the valve M is connected by a pipe 94 to one end of a cylinder 95 which forms the proportional part of the regulator 46. The second end of the cylinder 95 is connected by a pi e 53 to the space adjacent to the second end of the valve 91. The pistons in the cylinders 93 and 95 act through a two-arm lever 96 and a spring on a piston 97 moving in a cylinder connected by a pipe 9% to a motor operator g9 for actuating the feedwater control valve 12.

A water level control device 52 connected to the separator 14 which is placed downstream of the heating section 3' actuates the blowdown valve 55 in the blowdown pipe 54. The Water level control device also actuates a cam 160 forming part of the proportional (P) regulator 53. The configuration of the active surface of the cam 1% corresponds to the flow area of the valve 55. The cam 109 is followed by a roller at the end of a rod 101 which is connected by means of a spring MP3 to a piston 162. The latter is movable in a cylinder connected by a conduit 56 to a pilot valve 104 forming part of the integral (1) device 57. The pilot valve 104 is connected to a cylinder N5 in the same manner as the pilot valve hit of the regulator 46' is connected to the cylinder 93. A piston in the cylinder 1&5 is connected by a piston rod and a spring 107 to a piston 1th) movable in a cylinder which is connected to the pipe 58.

The pilot valve 104 is not only controlled by the pressure in the pipe 56 but also by the pressure in the pipe so which is connected to a cylinder wherein a piston 169 is movable. The piston Th9 forms part of the rate of flow measuring device 34 combined with the correcting device 34. The rate of how measuring device cornprises a restrictor litl interposed in the pipe connecting the steam space of the separator 14 to the first superheater sections 4. The pressures upstream and downstream of the restrictor iii? are conducted through pipes 111 and to a pressure difference measuring device 112. The latter actuates a rockable cam 114 by means of a linkage H3. The cam 114 is followed by a rod 115 whose movements are transmitted through a spring 103 to the piston 189. The active surface of the cam 114 is shaped to transform the measured pressure difference to a value corresponding to the rate of steam flow and to so correct the value that the pressure in the pipe 6t) can be used as set point pressure for the operation of the blowdown valve 55 as required when starting and cooling off the steam generator. The control pressure in the pipe 56 corresponds to the actual rate of blowdown.

A second cam 117 is connected through a linkage 116 to the first cam 114. The cam 117 acts on a piston 118 in the same manner as the cam 114 acts on the piston 109. The earn 117 is so shaped as to transform the measured pressure difference to a value corresponding to the rate of steam flow without the correction embodied in the cam 114. The piston 118 is movable in a cylinder which is connected by a pipe 119 to the motor operator 99 of the feed valve 12.

The rod 79 forming part of the change-over device 28 and connected to the lever 77 is also connected to a twoarm lever 12%; one arm of the latter is elastically connected to the stem 122 of a valve 121 interposed in a pipe 123 and controlling the supply of pressure fluid to the pilot valve 1%. The second end of the two-arm lever 12% is connected to an arm 125 controlling a twoway valve 126 interposed in the pipe 58 between the integration device 57 and the PI-regulator 4'6. Depend ing on the position of the arm 125 the valve 126 effects connection of the devices 57 and 46', or connection of the PI-regulator 46' by means of a pipe 61 to a set point transmitter 127. The latter includes a piston 128 controlling the fiow of a pressure fluid to and from a cylinder wherein the piston 12% is movable and to and from the pipe 61. The position of the piston 128 in its cylinder can be changed by manipulating a hand wheel 129 to perature in the tube 63 the lever 65 is moved clockwise, causing an increase of the pressure of the pressure fluid in the cylinder controlled by the piston 68 and in the pipe 69 so that the piston 71 is moved downward, the

downward movement being transmitted through the elements 73, 77, 78, 84 and 85 to the piston 86, increasing the pressure in the pipe 90. The increased pressure in the pipe 90 acts on the pilot valve 91 and on the piston in the cylinder 95 so that both move downward. The downward movement of the piston in the cylinder 95 causes a downward movement of the piston 97, reducing the pressure in the pipe 98 so that the piston of the motor operator 99 moves downward. Downward movement of the pilot valve 91 eifects supply of pressure fluid to the top side of the piston in the cylinder 93, moving said piston downward and assisting the effect of the downward movement of the piston in the cylinder 95. Downward movement of the piston 99 causes opening of the feed valve 12. This efiect lasts until the pressure acting on the valve 91 and on the piston in the cylinder 95 is equal to the set point pressure which is applied by the pressure fluid in the pipe 58 to the undersides of the pilot valve 91 and of the piston in the cylinder 95.

The set point pressure is obtained by a comparison of the blowdown rate and the rate of flow of the steam from the separator 14 to the superheater 4 which rate of flow is transformed by the cam 114. Control pressures corresponding to the rate of blowdown and to the transformed rate of steam flow act on the pilot valve 104 of the integration device 57. The pressure in the pipe 60 which corresponds to the transformed rate of steam flow forms the set point for the pressure in the pipe 56 for controlling the rate of blowdown. If the pressure in the pipe 56 is greater than that in the pipe 60 the valve 104 is moved to the right whereby pressure fluid is conducted from the supply pipe 123 to the right side of the piston in the cylinder 105, moving the piston therein to the left. This increases the pressure on the spring 107 and the pressure in the cylinder for the piston 106 and in the pipe 58, moving the piston 91 and the piston in the cylinder 95 upward whereby also the lever 96, the piston 97 and the piston in the motor operator 99 are moved upward, reducing the flow area of the feed valve 12. The control pressure corresponding to the rate of steam flow in the superheater section 4 and produced by the cam 117 and prevailing in the pipe 119 acts as an advance signal on the motor operator 99.

When the plant is in normal operation and the pressure of the operating medium corresponds to its critical pressure the piston 80 is close to the right end of the cylinder wherein it is movable and the rod 79 has moved the lever 77 and the slide 76 to the right as indicated by a dotted line in FIG. 3. The pressure at which the piston changes its position can be adjusted by manipulation of a set point value setting device 131 which is constructed in the same manner as the set point transmitter 127. -When the lever 77 is in the right end position, the feedwater supply is controlled in response to the temperature measuring device downstream of the first superheating section 4 in the same manner as the feedwater supply is controlled by the temperature measuring device 56 during start-up and cooling down of the steam generator. The lever 12% is simultaneously rocked clockwise with the lever 77, causing closing of the valve 121 and stopping of pressure fluid supply to the valve 104 of the integration device 57 and actuation of the two-way valve 126 to transmit the regulating pressure produced in the transmitter 127 through the pipe 61 to the pipe 58 and to the pilot valve 91 and to the piston in the cylinder of the PI-regulator 46. Under these operating conditions the PI-regulator 46' is not connected to the cylinder for the piston 166.

The change-over from the starting-up operation to normal operation and from normal operation to cooling down and stopping of the steam generator may be effected by electrical means, for example, as shown in FIG. 4. A

wiper of a potentiometer 151 is connected to the rod 79 which is actuated by the piston 30. The ends of the resistor 151 are connected to electric conduits 152 and 153 which carry voltages corresponding to the temperature at the outlet of the tube 63 and between the first and second superheater sections, respectively. A conduit 154 is connected to the wiper 150 and to a conventional electric PI-regulator 46. Depending on the position of the wiper 150 either the voltage of the conduit 152 or of the conduit 153, or an intermediary voltage is transmitted through the conduit 154 to the PI-regulator which regulates the rate of feedwater flow to the steam generator accordingly. The two-way valve 126 may be similarly controlled.

The steam power plant shown in FIG. 5 does not comprise a feedwater regulating valve as used in the steam power plant shown in FIGS..1 and 2. A conventional hydraulic variable speed transmission 161 is interposed between feed pump 10 and an electric motor 169 driving the pump. The speed of rotation of the feed pump 10 is controlled by a signal produced by the PI-regulator 46" and conducted to the variable speed transmission 161 through a conduit 162 for adjusting the speed of the motor.

A pipe 163 is connected to the feed pipe 13 between the feed pump 10' and the economizer 2. The pipe 163 is connected by a pipe 164 to a pipe connecting the second superheater section 5 to a third superheater section 6 and by a pipe 165 to a pipe connecting the third superheater section 6 and a fourth superheater section 6 for injecting liquid operating medium into the superheated steam for regulating the temperature thereof. The amount of injected liquid operating medium is controlled by valves 166 and 167. Temperature (t) measuring devices 168 and 169 are placed downstream of the third and fourth superheater sections, respectively, for producing signals which are conducted to PI-regulators 170 and 171, respectively. The output sides of these regulators are connected by conduits 172 and 173 to the valves 166 and 167, respectively. The temperature measuring device 168 is not only connected to the PI-regulator 170 but also to a P-regulator 174- whose output signal is superimposed on the output signal of the PI-regulator 171 as indicated by plus signs in the drawing. Similarly, the output signal of a P-regulator 175 is superimposed on the output signal of the PI-regulator 170 as indicated by plus signs in the drawing, the output signal of the P-regulator 175 corresponding to the temperature of the steam between the superheater section 5, and the water'injection (m) from the pipe 164 is measured by a device 179. The set point of the PI-regulator 170 is influenced by a signal corresponding to the rate of flow (111) of injection water through the pipe 165 and measured by'a device 178; the signal is smoothed by an integral element 177. The rate of flow measuring device 179 produces signals which are conducted through a conduit 130 to an I-element 181. The output signal of the I-element 181 is added at 182, as indicated by plus signs, to a signal in the conduit 41 produced by a temperature measuring device 40 interposed between the heating surface sections 4 and 5 and producing the control signal for regulating the feedwater supply to the steam generator at normal operation, The sum of the signals in the conduit 41 and emanating from the I-element 181 is conducted through a conduit 44 to ensue /s the change-over device 23. The tubular heating section 3 of the steam generator is provided with a multithermostat 29 as in FIG. 1, the output signal (i of the thermostat 29 being conducted through conduit to an addition point 183. A signal corresponding to the moisture content of the steam leaving the section 3 is superimposed. on the temperature signal produced in the device 29. The signal corresponding to the moisture content of the steam is obtained by combining a signal corresponding to the blowdown rate and a signal corresponding to the rate of feedwater supply to the steam generator. The so produced signal is smoothed in an I-element 134. The rate of flow (In) in the blowdown pipe 54 is measured by an apparatus 1W5 which produces a signal conducted through conduit 13*) to a comparison and transforming element 186 which is also connected by a signal conduit 187 to the signal conduit 21' of the rate of feedwater flow (at) measuring device 2%. The output of the element 1% is conducted into the I-element tra l through a conduit A signal conduit 33 interconnects the signal addition point 183 and the change-over device whose utput side is connected to the FRI-regulator 46", supplya set point signal thereto. The regulator receives and is also responsive to the signals arriving from the rate of fcedwatcr flow measuring device 2%) through the signal conduit The T l-regulator as" is further influenced by a signal produced by a rate or flow (m) measuring device 34 interposed between a separator 14 and the first super-heater section 4. The signal produced the device 34 acts as a preimpulse in the rate of feedwater supply control cycle and immediately increases the feedwater supply upon a sudden increase of steam consumption and conversely.

When the steam generator is started the rate of feedwater supply is controlled in response to the temperature at the outlet of the heating section 3 because, due to the subcritical pressure (p) of the operating medium measured by the device 24, the change-over device 23 connects the multithermostat 29 to the Pl-regulator 46". At normal operation of the plant the pressure responsive device 24 places the change-over device 23 in the position connecting the PI-regulator 46" to the temperature measuring device 46 and the feedwater regulation is responsive to the temperature of the steam between the heating surfaces 4 and 5. The signal produced by the device 41 is corrected by a signal corresponding to the rate of flow of injection water in the pipe 164 which signal is smoothed in the I-element 181. The signals produced in the rate of flow measuring devices 1'78 and 179 and smoothed in the I-elernents 177 and 181 affect the set point of the PI-regulator 1'76) and the signal produced by the temperature measuring device 46, respectively. The result is that operations made necessary by the desired temperature changes of the steam are performed as far as possible within a zone where the temperature of the operating medium is relatively low so that in the high temperature zones of the steam generator only small quantities of injection water are required for correction of the steam temperatures and the thermodynamic losses are reduced.

As in the examples shown in FIGS. 1 and 2, the plant according to FIG. 5 may be provided with at least one steam-heated feedwater preheater which is interposed in "it? the feed pipe between the feed pump 10 and the economizer 2.

I claim:

A forced flow steam generator constructed and arranged so as to selectively produce steam at subcritical and at supercritical pressure, comprising:

a tubular evaporating section wherein water is evaporated,

a tube system connected to said evaporating section for receiving operating medium therefrom and having an outlet,

said evaporating section having an inlet,

control means connected to said inlet for controlling the rate of feeclwater supply to said tubular evapo rating section,

first temperature responsive means connected substantially to the end of said tubular evaporating section and being responsive to the temperature of the operating medium flowing thereat,

second temperature responsive means connected to said tube system upstream of the outlet thereof and being responsive to the temperature of the operating medium flowing within and upstream of the outlet of said tube system,

a change-over device for selectively connecting said first emperature responsive means and said second temperature responsive means to said control means for selectively controlling the rate of feedwater supply either in response to the temperature of the operating medium substantially at the end of said evaporating section or in response to the temperature of the operating medium flowing within said tube system, and

means responsive to the pressure of the operating medium in the steam generator and operatively connected to said change-over device for actuating the latter to connect said first temperature responsive means to said control means when the pressure of the operating medium is below the critical pressure of water and to connect said second temperature responsive means to said control means when the pressure of the operating medium is above the critical pressure.

References Cited by the Examiner UNITED STATES PATENTS 1,975,104 10/34 Iunldns 122451 2,804,851 9/57 Smoot.

2,869,517 1/59 Lieberherr l22406 2,921,441 1/60 Buri.

2,966,896 1/61 Vogler.

2,984,984 5/61 Dickey.

FOREIGN PATENTS 859,189 1/61 Great Britain.

OTHER REFERENCES A Supercritical Pressure Plant, Combustion, August 1956, pages 47-56 (only page 54 relied upon).

PERCY L. PATRICK, Primary Examiner.

FREDERICK L. MATTESON, ]R.,

KENNETH W. SPRAGUE, Examiners, 

